Method for precipitating iron from a zinc sulphate solution as hematite

ABSTRACT

This invention relates to a method for the removal of iron as hematite from a zinc sulphate solution in atmospheric conditions during the electrolytic preparation of zinc. According to the method, the pH of the iron-containing solution is adjusted to a value of at least 2.7, oxygen-containing gas is fed into the solution and part of the hematite thus formed is recirculated to the precipitation stage.

This invention relates to a method for the removal of iron as hematitefrom a zinc sulphate solution in atmospheric conditions during theelectrolytic preparation of zinc.

Zinc calcine, obtained by roasting sulphidic zinc concentrates, isgenerally used as the starting material in the electrolytic preparationof zinc. The main component of the calcine is zinc oxide, ZnO, but someof the zinc is also bound to iron in the form of zinc ferrite ZnO.Fe₂O₃.The amount of zinc ferrite is usually so considerable that recoveringthe zinc from it is unavoidable. Zinc oxide is easily soluble even athigh pH values, whereas the ferrite has to be leached at a higher acidcontent. Ferrite leaching is performed in a separate stage, where bothzinc and iron are dissolved. The iron has to be precipitated from thesolution obtained before the solution can be returned to the neutralleach and from there to zinc sulphate solution purification andelectrolysis. The above process is described e.g. in U.S. Pat. Nos.3,434,947 and 3,493,365.

In industrial processes zinc oxide leaching, a neutral leach, isgenerally carried out at a pH of 2-5 and ferrite leaching at an acidcontent of between 30-100 g H₂SO₄/l. The solution from ferrite leaching,which contains the dissolved zinc and iron, is very acidic, and is oftenpre-neutralised before the iron is precipitated from it. Ferriteleaching can also be combined with the iron precipitation stage. Thismethod is known as the conversion process and is described in U.S. Pat.No. 3,959,437.

Nowadays, the leaching of zinc concentrate is also combined with zincoxide or calcine leaching in ever greater amounts. The concentrate isfed either to ferrite leaching or is leached as a separate pressureleach. The main component in concentrate leaching is zinc sulphide, ZnS.In this case too, some of the zinc is bound to zinc ferrite. Inaddition, the iron in the concentrate is bound to pyrite FeS₂, and someof the zinc of the zinc sulphide may be replaced by iron. For thisreason an iron removal stage is also needed in a zinc process that isbased on concentrate leaching or includes a concentrate leaching stage.

The zinc content of the zinc sulphate solution going to ironprecipitation is usually of the order of 120-180 g/l. Depending on theprocess used, the amount of trivalent ferric iron in the zinc sulphatesolution varies from a few grams up to dozens of grams per litre. Threeiron precipitation processes are in use and in them the iron isprecipitated as either jarosite Na[Fe₃(SO₄)₂(OH)₆], goethite FeOOH orhematite Fe₂O₃.

When iron is precipitated as jarosite or goethite, a neutralising agentis to be used in precipitation to neutralise the sulphuric acid releasedin the reactions. Normally the neutralising agent is calcine.

A conventional jarosite process is described in the above-mentioned U.S.Pat. No. 3,434,947, whereby iron is precipitated at a temperature closeto the boiling point. The free acid is neutralized to a value of 3-5 g/lH₂SO₄ (optimal pH 1.5). The amount of iron in the zinc sulphate solutionis 20-35 g/l. In order for the jarosite to obtain an essentiallycrystalline form, which has beneficial settling properties, potassium,sodium or ammonia ions are also added to the solution.

Goethite precipitation is described for instance in U.S. Pat. No.4,676,828. In this method, the amount of free acid in the zinc sulphatesolution going to iron precipitation is 4-8 g/l and the amount of ferriciron 1-2 g/l. Oxygen and calcine are fed into the solution, so that theiron is precipitated as goethite.

When iron is precipitated as hematite according to the conventionalmethod, it is performed from a solution, from which the iron is firstreduced from trivalent to divalent form. After this, the iron isprecipitated hydrolytically by oxidation without neutralisation:2FeSO₄+O₂(g)+2H₂O==>Fe₂O₃+2H₂SO₄   (2)The precipitation of iron must however be performed in an autoclave attemperatures of about 200° C. with a partial oxygen pressure of about 18bar, which has essentially restricted the adoption of the method, eventhough hematite is in fact the most environmentally friendly form ofiron precipitate.

Now it has surprisingly been discovered that, with the suitableadjustment of conditions, iron can be precipitated as hematite from aniron-containing zinc sulphate solution in atmospheric conditions too.The term atmospheric conditions here means conditions where the reactorsused in the precipitation stage are unpressurised and the temperature ofthe zinc sulphate solution is regulated to between 80° C. and theboiling point of the solution. According to the method now developed,the pH of the solution is neutralised in the iron precipitation stage toa value of at least 2.7 and oxygen is fed into the solution or in theform of oxygen or oxygen-containing gas. During precipitation thereshould also be some hematite nuclei in the solution, i.e. theprecipitate generated in precipitation is recirculated from the end ofthe precipitation stage back to the beginning. The essential features ofthe invention will be made apparent in the attached claims.

The sulphuric acid content of the zinc sulphate solution going to ironprecipitation depends on the process used. Zinc sulphate solution fromferrite leaching is generally rather acidic, with perhaps 10-40 gsulphuric acid/l. If the solution comes from a concentrate leachingstage its sulphuric acid content may be a little lower. For hematiteprecipitation the solution must be neutralised before being fed to theiron precipitation stage. The sulphuric acid generated during hematiteprecipitation must also be neutralised in order to keep the pH stable.Neutralisation may be performed using any appropriate neutralisingagent. Tests carried out show that the pH of the solution at the startof precipitation should be at least 2.7. The neutralising agentgenerally used in goethite and jarosite precipitation is calcine. Inaddition to calcine, calcium compounds are used as neutralising agents,and hydroxide and ammonia compounds such as sodium hydroxide are alsoeffective neutralising agents.

The hematite precipitation reaction also requires oxygen, and so oxygenis fed to precipitation either as oxygen or in the form of anoxygen-containing gas such as air.

In hematite precipitation occurring in atmospheric conditions, it isessential that the hematite precipitate generated is recirculated to thestart of the precipitation stage as hematite nuclei. Recirculationoccurs for instance by routing the hematite precipitate obtained fromthe liquid-solid separation step after the precipitation stage back tothe beginning of the process as seed nuclei for the new precipitate.Preferably at least ⅕ of the precipitate is recirculated to thebeginning of the precipitation stage.

We have also found that thorough mixing of the neutralising agent andthe recirculation hematite precipitate and a good, controlled dispersionof the oxygen-containing gas for the precipitation stage of the reactorslurry in both reactors have a beneficial effect on the iron beingprecipitated as hematite. One advantageous method is to use a mixer withlow shear force but a diameter of over half that of the mixing reactor.One such mixer is described in U.S. Pat. No. 5,182,087, where thediameter of the mixer is at least 0.7 times that of the reactor.

The precipitation of iron as hematite, particularly when it can beperformed in atmospheric conditions, is worthwhile in many respects.Firstly, the iron content of hematite is known to be double that ofjarosite for example. This means that the amount of waste generated isabout half the corresponding amount of jarosite. The zinc content ofhematite is far below the level of zinc in jarosite and goethite, sothat zinc recovery yields are improved. The table below presents acomparison between various process alternatives when hematite is formedin an autoclave (Okada, S. et al: “Zinc residue treatment at lijimaRefinery”, TMS of AIME, Paper No. A84-51, p. 8):

Hemalite Jarosite Goethite Generated α-Fe₂O₃ Me—Fe₃(SO₄)₂(OH)₆ α-FeOOH,compound β-FeOOH Fe, wt %  50-60 30-35 40-45 Zn, wt % 0.5-1.0  3-5  2-3Amount (dry) 1.8 t/t Fe 3.1 t/t Fe 2.4 t/t Fe Amount (wet) 2.0 t/t Fe4.8 t/t Fe 4.0 t/t Fe Moisture content 10% 35% 40%

When researching the method of the present invention, it was found thathematite can be made to precipitate directly onto the hematite nucleiwhen the conditions are correct, as described above. If the conditionsare not exactly right, precipitation may also occur partially asjarosite or goethite.

The precipitate obtained from the precipitation stage is routed to thenormal solids separation, from where the iron-free zinc sulphatesolution obtained is fed to the neutral leaching stage and the hematiteprecipitate either to the waste area or for further processing.

Many processes that are usually performed at increased pressure andtemperatures, also work in atmospheric conditions, but the reactionsrequired in the processes proceed so slowly that the method cannot beadopted. Research has now shown, however, that iron can be precipitatedas hematite in atmospheric conditions and that the precipitation time iseven shorter than for instance in jarosite precipitation. Thus it hasbeen possible to precipitate the iron of a zinc sulphate solution ashematite in favourable conditions in an average of 3-6 hours.

The invention is further described with the aid of the followingexamples.

EXAMPLE 1

Iron was precipitated from a zinc sulphate solution in batch tests usinga 5 l container as the mixing reactor and a gls mixer that disperses gaswell, as described e.g. in U.S. Pat. No. 4,548,765. The pH of the zincsulphate solution was raised with NaOH from a value of <1 to a value of3.1 and kept at this value and at a temperature of 93° C. The solutioncontained about 120 g zinc/l and about 9.3 g iron/l. About 55 g/l ofpure hematite was used as hematite nuclei. After 5.5 hours the iron hadalmost completely precipitated from the solution, and the final ironcontent in the solution was 65 mg/l. Thus the iron content of thebatch-tested precipitate was 70 wt % and the zinc content about 0.5%.The hematite composition of the precipitate was confirmed by X-raydiffraction (XRD) analysis.

EXAMPLE 2

An iron precipitation test was carried out on a zinc sulphate solutionin a continuous testing apparatus. The apparatus consisted of threemixing reactors, each of which having a volume of 5 l, and a thickener,also 5 l in volume. The temperature of the reactors was regulated at 96°C. and the pH at 3.0. The zinc sulphate solution, with an iron contentof about 9.5 g/l and the pH raised to a value of 3.0, was fed into thefirst mixing reactor at a rate of 1000 ml/h. A spiral-type mixer wasused as the reactor mixer, with which an even mixing was achievedthroughout the whole solution volume. About 75% of the underflow of themixer was recirculated back and the rest was taken out as hematiteprecipitate. The iron content of the overflow solution from thethickener was around 500 mg/l and the iron content of the underflow 52%.XRD analysis showed the precipitate to be hematite.

As shown by the examples above, iron precipitation from a zinc sulphatesolution as hematite also works in atmospheric conditions, when theconditions are made favourable. In the batch test the iron content inthe precipitate was even higher (70%) than the iron content reported inautoclave precipitation (50-60%), but the continuous test also reachedthe iron content levels of precipitate prepared in an autoclave. Theprecipitate obtained had good filtration properties.

1. A method for the removal of iron as hematite from a zinc sulphatesolution containing iron, comprising performing the iron precipitationstage in atmospheric conditions by using unpressurised reactors, keepingthe temperature of the zinc sulphate solution in the region of 80°C.-96° C., feeding a neutralising agent in order to raise the pH of thezinc sulfate solution to at least 2.7 and an oxygen-containing gas intothe solution, and carrying out the precipitation out in the presence ofhematite nuclei.
 2. A method according to claim 1, wherein theneutralising agent is calcine.
 3. A method according to claim 1, whereinthe neutralising agent is a calcium compound.
 4. A method according toclaim 1, wherein the neutralising agent is a hydroxide compound.
 5. Amethod according to claim 1, wherein the neutralising agent andoxygen-containing gas are fed into the precipitation stage reactorsevenly throughout the solution volume of the reactors.
 6. A methodaccording to claim 1, wherein at least ⅕ of the hematite generated isrecirculated as hematite nuclei.
 7. A method according to claim 1,wherein the oxygen-containing gas is oxygen.
 8. A method according toclaim 1, wherein the oxygen-containing gas is air.